CDMA communications method and system

ABSTRACT

A CDMA communications method capable of multiplex transmission of data over a wide range from a low rate to high rate, such as image data, without a considerable increase in a circuit scale. A fundamental transmission rate is determined, for example, at 32 kbps, a rate higher than 8 kbps conventionally used. The data whose transmission rate is equal to the fundamental transmission rate is transmitted in frames including no vacant portion. Data whose transmission rate is lower than the fundamental transmission rate (16 kbps, for example) are transmitted in frames including vacant portions. The vacant portions are not transmitted. This makes it possible to receive data through other channels during a time period associated with the vacant portions. Data of a higher transmission rate, 128 kbps, for example, can be multiplexed and transmitted through four channels using different spreading codes.

This is a divisional of application Ser. No. 08/335,089, filed on Nov.7, 1994 now U.S. Pat. No. 5,586,113.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a CDMA (Code Division Multiple Access)communications method and system preferably applied to mobilecommunications.

2. Description of Related Art

CDMA communication systems carry out a primary modulation of originaldata to be transmitted by QPSK or the like, and a secondary modulationof the primary modulation signal to enlarge the bandwidth by a spreadingcode such as a PN code (Pseudo-Noise code). The bit rate of thespreading code is called a chip rate, which is several tens to severalhundreds times higher than the bit rate of the original data. Each userperforms communications using the same frequency band, and is identifiedby a spreading code. Conventional CDMA systems arrange the original datainto frames, perform the primary and secondary modulations of theframes, and transmit them. During communications, there are silentintervals, in which the modulations are stopped, and the transmission isinterrupted. This prevents useless radio waves from being emitted,thereby suppressing interference power to other mobile stations. On theother hand, data whose transmission bit rate is less than that of voicecoded data is transmitted by making vacant positions in a bit series ina frame, and stopping modulation of the vacant positions. The modulationis interrupted in either case. However, since the interruption iscarried out in a random manner, the receiving side cannot utilize thisvacant time for other purposes such as receiving broadcastinginformation transmitted from base stations.

Data rates of voice codecs for cellular systems are ranging from 8-16kbps. Low rate data (e.g., 1.2-4.8 kbps) as well as facsimile data arealso handled as important cellular services. Furthermore, it will becomenecessary in the near future to transmit signals of the ISDN (IntegratedService Digital Network) in addition to the low bit rate datatransmission. Image codecs currently available generally employtransmission rates of 64 kbps or 384 kbps. To achieve flexibletransmissions of low rate data to high rate image data, the multimediatransmission including images requires code division multiplexing in theCDMA systems.

FIGS. 1A-1C illustrate a code division multiplexing method when a highrate transmission is carried out in a conventional CDMA system. FIG. 1Ashows a fundamental channel whose fundamental transmission rate is f_(b)bps. A frame includes a synchronizing word SW, and a traffic channelTCH. FIGS. 1B and 1C illustrate the frame arrangements when thetransmission rate is twice and four times as that of the fundamentaltransmission rate, respectively. When the transmission rate is twice,that is, 2f_(b) bps, two traffic channels are transmitted in parallel byusing two different spreading codes (which is referred to as atwo-channel parallel transmission) as shown in FIG. 1B, and when thetransmission rate is four times, 4f_(b) bps, the data is transmitted bythe four-channel parallel transmission as shown in FIG. 1C.

When the transmission rate of a coded voice signal is 8 kbps, forexample, and this transmission rate is adopted as the fundamentaltransmission rate of a fundamental channel, an 8-channel code divisionmultiplexing is required to transmit data at a transmission rate of 64kbps. Moreover, a 256-channel code division multiplexing is required totransmit data at a transmission rate of 2 Mbps.

The conventional CDMA systems determine the transmission rate of thecoded voice signal which is most frequently used as the fundamentaltransmission rate of the channel, and makes a frame corresponding to thefundamental transmission rate the fundamental frame. A high-speedtransmission such as multimedia transmission is performed by paralleltransmission of a plurality of fundamental channels at the transmittingside, and by discriminating the individual channels by correlators atthe receiving side. This presents a problem in that the circuit scale ofa transmitter and a receiver increases with the degree of multiplexing.

Furthermore, in the CDMA mobile communications, communication qualitywill be degraded as a mobile station moves apart from a base stationduring communications. Accordingly, to maintain a high qualitycommunication state, it becomes necessary to search for a new basestation close to the mobile station during the communication, and toconnect the mobile station to the new base station.

However, since a user occupies the entire frame for the communication,and there is no vacant time in the frame time for searching for a newbase station to be connected, it is impossible to detect the new basestation to which the mobile station switches the communication.Therefore, the detection must be performed at the base station siderather than the mobile station side. More specifically, both the basestation, which is communicating with the mobile station, and neighboringbase stations receive the signal from the mobile station, measure thepower of the received signal, and select the base station whose receivedsignal has the maximum power as the new base station. This, however,presents a problem in that an amount of processing at the base stationside increases with the increase in the number of the mobile stations.On the other hand, to perform this detection at the mobile station side,the mobile station must be provided with another secondary demodulationsystem (or decorrelator) for measuring the power of the control channelstransmitted from neighboring base stations.

Each base station continuously or periodically transmits, through acommon control channel, position information of the base station andspreading codes used by the neighboring base stations, as well ascommunication data to mobile stations. In addition, broadcasting datasuch as weather forecasts, stock prices, and paging data can also betransmitted. To receive these data (broadcasting data or paging data)during the communication, a conventional mobile station requires anotherdemodulation system in addition to the demodulation system for theoriginal communications.

Providing two demodulation systems in the mobile station presents aproblem in that it will increase size, weight, and consuming power ofthe mobile station.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a CDMAcommunications method and system which can implement variable rate datatransmission from a high-rate to low-rate with a small amount ofincrease in circuitry of transceivers.

Another object of the present invention is to provide a CDMAcommunications method and system which can detect a base station, towhich a mobile station is to be newly connected, at the mobile stationside during the communication.

Still another object of the present invention is to provide a CDMAcommunications method and system wherein a mobile station can receivevarious channel data transmitted from base stations while the mobilestation is communicating.

In a first aspect of the present invention, there is provided a CDMA(Code Division Multiple Access) communications method for transmittingtransmission data through one or mote channels between base stations anda mobile station, the method comprising the steps of:

generating a frame including at least a part of the transmission dataand a vacant portion when a transmission rate of the transmission datais lower than a predetermined rate, the vacant portion having no data tobe transmitted;

performing a primary modulation of the frame to produce a primarymodulation signal;

performing a secondary modulation of the primary modulation signal usinga spreading code to produce a secondary modulated wideband signal, thespreading code being different for each of the channels; and

transmitting the wideband signal using a carrier.

Here, the step of generating a frame may comprise the steps of:

predetermining a transmission rate higher than a minimum transmissionrate as a fundamental transmission rate; and

determining a frame length such that the frame includes no vacantportion when data is transmitted at the fundamental transmission rate.

The minimum transmission rate may be a rate of transmitting a voicesignal.

The CDMA communications method may further comprise the step ofgenerating a plurality of frames to be assigned to a plurality of thechannels when data is transmitted at a rate higher than the fundamentaltransmission rate.

The step of generating a frame may comprise the step of time-compressingthe transmission data by a factor of N at every time period T (N is aninteger greater than one, and T is the length of a frame at thefundamental transmission rate) when the transmission rate of thetransmission data is 1/N of the fundamental transmission rate, therebyproviding the frame with the vacant portion.

The CDMA communications method may further comprise the steps of:

obtaining the primary modulation signal by receiving a wideband signalassociated with one of the channels, and by despreading the widebandsignal using a spreading code; and

restoring the transmission data by primarily demodulating the primarymodulation signal obtained at the step of obtaining, and bytime-expanding the demodulation output by a factor of N.

The CDMA communications method may further comprise the steps of:

measuring received power of a common control channel signal transmittedfrom a base station, other than a base station with which the mobilestation is communicating, by switching the spreading code during a timeperiod corresponding to the vacant portion in the frame; and

deciding, during communications, a base station to which thecommunication is to be switched in accordance with the received power.

The CDMA communications method may further comprise the steps of:

receiving common control channel data transmitted from at least one ofthe base stations, during a time period corresponding to the vacantportion of the frame, by switching the spreading code; and

demodulating the common control channel data.

The step of generating a frame may comprise the step of generating aframe from the transmission data by using a plurality of T/N longportions in the frame, the transmission data having a transmission rateequal to or less than (N-1)/N times of the fundamental transmission rateof the channel, where N is an integer equal to or greater than three.

In a second aspect of the present invention, there is provided a CDMA(Code Division Multiple Access) communications apparatus fortransmitting transmission data through one or more channels between basestations and a mobile station, the apparatus comprising:

means for generating a frame including at least a part of thetransmission data and a vacant portion when a transmission rate of thetransmission data is lower than a predetermined rate, the vacant portionhaving no data to be transmitted;

means for performing a primary modulation of the frame to produce aprimary modulation signal;

means for performing a secondary modulation of the primary modulationsignal using a spreading code to produce secondary modulated widebandsignal, the spreading code being different for each of the channels; and

means for transmitting the wideband signal using a carrier.

The means for generating a frame may comprise:

means for predetermining a transmission rate higher than a minimumtransmission rate as a fundamental transmission rate; and

means for determining a frame length such that the frame includes novacant portion when data is transmitted at the fundamental transmissionrate.

The minimum transmission rate may be a rate of transmitting a voicesignal.

The CDMA communications apparatus may further comprise means forgenerating a plurality of frames to be assigned to a plurality of thechannels when data is transmitted at a rate higher than the fundamentaltransmission rate.

The means for generating a frame may comprise means for time-compressingthe transmission data by a factor of N at every time period T (N is aninteger greater than one, and T is the length of a frame at thefundamental transmission rate) when the transmission rate of thetransmission data is 1/N of the fundamental transmission rate, therebyproviding the frame with the vacant portion.

The CDMA communications apparatus may further comprise:

means for obtaining the primary modulation signal by receiving awideband signal associated with one of the channels, and by despreadingthe wideband signal using a spreading code; and

means for restoring the transmission data by primarily demodulating theprimary modulation signal obtained by the means for obtaining, and bytime-expanding the demodulation output by a factor of N.

The CDMA communications apparatus may further comprise:

means for measuring received power of a common control channel signaltransmitted from a base station other than a base station with which themobile station is communicating, by switching the spreading code duringa time period corresponding to the vacant portion in the frame; and

means for deciding during communications a base station to which thecommunication is to be switched in accordance with the received power.

The CDMA communications apparatus may further comprise:

means for receiving common control channel data transmitted from atleast one of the base stations, during a time period corresponding tothe vacant portion of the frame, by switching the spreading code; and

means for demodulating the common control channel data.

The means for generating a frame may comprise means for generating aframe from the transmission data by using a plurality of T/N longportions in the frame, the transmission data having a transmission rateequal to or less than (N-1)/N times of the fundamental transmission rateof the channel, where N is an integer equal to or greater than three.

According to the present invention, since a transmission rate greaterthan the widely used voice transmission rate (8 kbps, for example) isselected as the fundamental transmission rate, the number of channels tobe multiplexed is reduced, as compared with that of the conventionalsystems, when data of a higher transmission rate is transmitted. As aresult, the circuit scale of a multiplexing circuit and that of adivision circuit can be reduced. In addition, although a datatransmission requires the entire channel even if its transmission rateis less than the fundamental transmission rate, the average transmissionpower is reduced because only part of the frame is used in this case,and hence, the interference power to other channels are reduced. As aresult, capacity in terms of the number of subscribers of the system, inaccordance with the present invention, is the same as that of theconventional CDMA system using a minimum transmission rate as thefundamental rate. In other words, the present invention, which adopts arate higher than the minimum transmission rate as the fundamental rate,is not inferior to the conventional system in capacity in terms of thenumber of subscribers.

In addition, since a vacant portion is provided in each frame when atransmission rate is lower than the fundamental transmission rate, amobile station can receive, during the communication, the common controldata from other base stations by switching spreading codes in the vacantportion. Furthermore, comparing the received powers of common controlchannels transmitted from neighboring base stations makes it possible todetermine the base station to which the mobile station is to be newlyconnected. In this case, since the two demodulation systems, which arerequired in the conventional mobile station, can be replaced with asingle demodulation system, the increase in hardware is small.

The above and other objects, effects, features and advantages of thepresent invention will become more apparent from the followingdescription of the embodiments thereof taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C are schematic diagrams illustrating channel arrangements ofa conventional parallel transmission method at various transmissionrates;

FIGS. 2A-2D are schematic diagrams illustrating channel arrangements ofa parallel transmission method, in accordance with the presentinvention, at various transmission rates;

FIG. 3 is a block diagram showing an embodiment of a transmitter of abase station, to which the parallel transmission method, in accordancewith the present invention, is applied;

FIG. 4 is a block diagram showing an embodiment of a receiver, to whichthe parallel transmission method, in accordance with the presentinvention, is applied;

FIG. 5 is a block diagram showing a base station, to which the CDMAcommunications method, in accordance with the present invention, isapplied;

FIGS. 6A-6C are schematic diagrams illustrating an example of formingtraffic packets at the base station of FIG. 5;

FIG. 7 is a block diagram showing a mobile station, to which the CDMAcommunications method in accordance with the present invention, isapplied;

FIGS. 8A-8E are schematic diagrams illustrating receiving operation atthe mobile station of FIG. 7; and

FIGS. 9A-9F are schematic diagrams illustrating another example offorming traffic packets at the base station of FIG. 5.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will now be described with reference to the accompanyingdrawings.

FIGS. 2A-2D show the idea of the CDMA transmission method in accordancewith the present invention. In the present invention, a fundamentaltransmission rate f_(b) bps is determined such that it is a few timesgreater than a voice transmission rate, which has been conventionallyused as the fundamental transmission rate. For example, a transmissionrate of 32 kbps, which is four times greater than the conventionaltransmission rate of 8 kbps, is selected as the fundamental transmissionrate. A frame of the fundamental transmission rate includes asynchronizing word SW, and a traffic channel TCH. A spectrum-spreadwideband signal is generated by increasing the fundamental transmissionrate by a factor of the processing gain (bandwidth spreading factor)through the primary and secondary modulations.

When the transmission rate of data is lower than the fundamentaltransmission rate f_(b) bps, such as a low transmission rate codedvoice, the original traffic channel TCH, in a frame of the fundamentaltransmission rate, is thinned out as shown in FIGS. 2B and 2C, where thelength of a frame is indicated by T. FIGS. 2B and 2C illustrate caseswhen data are transmitted at a transmission rate (f_(b) /2) bps, or halfthe fundamental transmission rate f_(b) bps. In FIG. 2B, the latter halfof the traffic channel TCH is emptied, and in FIG. 2C, the trafficchannel TCH is divided into eight portions, and portions at evenpositions are cleared. These portions in the frame, other than thoseused to transmit data, are called vacant portions, in which no data aretransmitted. The vacant portions can be arranged in many other ways, aswell.

In the CDMA method which transmits a signal after spreading it into awideband signal by a PN (Pseudo-noise) code or a Gold code, thecapacity, in terms of the number of subscribers per unit bandwidth, isdetermined by interference power, including noise power. In thisembodiment, since data whose transmission rate is lower than thefundamental transmission rate are transmitted by thinning out thetraffic channel TCH in a frame to make vacant portions, and a signal isnot transmitted in the vacant portions, the interference on the otherchannels is reduced. For example, the interference power on the otherchannels, due to the transmission signal of FIGS. 2B and 2C, is reducedto half the power of a frame which is not thinned out. As a result, thecapacity, in terms of the number of subscribers can be doubled. In otherwords, the number of subscribers is automatically increased at a lowtransmission rate as compared with the number of subscribers at thefundamental transmission rate. In this case, the fundamentaltransmission rate is twice the coded voice rate, and hence, the systemof the present invention requires a double bandwidth of the conventionalCDMA system. Accordingly, the capacity is the same as that of theconventional system. No disadvantage is seen.

On the other hand, when the transmission rate of data is higher than thefundamental transmission rate f_(b) bps, such as 4f_(b) bps as shown inFIG. 2D, for example, a plurality of (four in this case) differentspreading codes are used to carry out the parallel transmission ofindividual channels. In this case, since the fundamental transmissionrate is set higher than the conventional one (four times higher in thiscase), the number of channels required is reduced from 16 channels to 4channels. Since the number of channels of the parallel transmission isreduced, the scale of a modulation circuit at the transmission side, andthat of a demodulation circuit at the receiving side, are reduced ascompared with those of the conventional system.

FIG. 3 shows an embodiment of a transmitter of a base station forimplementing the method in accordance with the present invention.

Each of n frame generation circuits 11₁ -11_(n) (n is a positiveinteger) generates frames, each of which includes a synchronizing wordand a traffic channel as shown in FIGS. 2A-2D, for each input data. Theframe generation circuit 11_(k) (k=1-n) has three input terminals: aninput terminal 12_(k), to which data of the fundamental transmissionrate f_(b) bps are inputted, an input terminal connected to the outputterminal of a TCH (traffic channel) frame thin-out circuit 13_(k), andan input terminal connected to a k-th one of the n output terminals of aserial-to-parallel converter 14 which converts data, whose transmissionrate is if_(b) bps (i is an integer greater than one and equal to orless than n), into i parallel data.

Each TCH frame thin-out circuit 13_(k) has an input terminal 15_(k), towhich data of an f_(b) /m bps (m is an integer greater than 1)transmission rate are applied, and converts the input data intothinned-out TCH data, which are discretely inserted into the trafficchannel TCH of a frame in a time division fashion as shown in FIGS. 2Band 2C. Data of an if_(b) bps transmission rate are inputted to theserial-to-parallel converter 14 through an input terminal 16. The datasupplied to the input terminals 12₁ -12_(n), 15₁ -15_(n), and 16 arecontrolled at the preceding stage, so that only one of the three inputterminals of the frame generation circuit 11₁ -11_(n) is provided withthe input data.

The serial-to-parallel converter 14 receives data of an if_(b) bpstransmission rate, converts them into i sets of parallel data, each ofwhich has the fundamental transmission rate f_(b) bps, and distributeseach set to each one of the frame generation circuits 11₁ -11_(i).

The output of the frame generation circuit 11_(k) is supplied to aprimary modulation circuit 17_(k), and is converted into two signals(in-phase signal I and quadrature signal Q) in accordance with themodulation method (QPSK, for example). The two signals produced from theprimary modulation circuit 17_(k) are supplied to complex multiplier19_(k), where the two signals are each multiplied by a spreading codesupplied from a spreading code generator 18_(k), and are spectrumspread. The spreading code generators 18₁ -18_(n) generate spreadingcodes different from each other. All the I signals which are spectrumspread and outputted from the complex multipliers 19₁ -19_(n) are addedby an adder 21_(I), and all the Q signals which are spectrum spread andoutputted from the complex multipliers 19₁ -19_(n) are added by an adder21_(Q). The outputs of the adders 21_(I) and 21_(Q) are converted intoanalog signals by D/A converters 22_(I) and 22_(Q), respectively. Afterthat, the analog signals, which are passed through low-pass filters23_(I) and 23_(Q), are inputted to a quadrature modulator 24, whichquadrature-modulates an intermediate frequency signal from an oscillator25 by the input signals. The modulated signal is passed through aband-pass filter 26, amplified by an amplifier 27, inputted to a mixer28, and frequency-mixed with a carrier signal from an oscillator 29. Theoutput of the mixer 28 is passed through a bandpass filter 31,power-amplified by an amplifier 32, outputted to an output terminal 33,and is radiated from an unshown antenna as an electric wave.

FIG. 4 shows an embodiment of a receiver receiving a signal transmittedfrom the transmitter of FIG. 3. The received signal at an input terminal41 is passed through a bandpass filter 42, amplified by an amplifier 43,and is frequency-mixed with a signal from an oscillator 45 by a mixer44. The mixed output is passed through a bandpass filter 46, so that theintermediate frequency component is passed. The intermediate frequencysignal is amplified by an automatic gain control amplifier 47 to asignal whose level is substantially constant. The amplified output isconverted into baseband I and Q signals by a quadrature detector 48using a signal from an oscillator 49. The I and Q signals are passedthrough low-pass filters 51_(I) and 51_(Q), and inputted to A/Dconverters 52_(I) and 52_(Q), which convert them into digital signals,respectively. The outputs of the A/D converters 52_(I) and 52_(Q) aredivided into n signals by a signal distribution circuit 53_(I) and53_(Q), and are inputted to n matched filters 54₁ -54_(n), respectively.The matched filters 54₁ -54_(n) take correlations between the inputsignals and codes associated with n spreading codes at the transmitterside, and spectrum-despread the input signals. The respective matchedfilters separate multipath components having different time delays.

RAKE demodulators 55₁ -55_(n), receiving the output of the matchedfilters 54₁ -54_(n), weight and add the separated multipath componentscoherently, and demodulate the added result. The output of the RAKEdemodulator 55₁ is supplied to a rate conversion circuit 56 for datacommunications of the rate less than or equal to f_(b) bps. The rateconversion circuit 56 outputs the data in continuous mode. Thus, whenthe transmission rate of the received signal is f_(b) /m bps, where m isan integer equal to or greater than one, demodulated data is obtained atthe output terminal 59. On the other hand, when the transmission rate ofthe received signal is if_(b) bps, signals of the individual channels,each having a transmission rate of f_(b) bps, are converted into aserial signal of if_(b) bps by the parallel-to-serial converter 57, andthe serial signal is outputted from an output terminal 58. A mobilereceiver which provides data communications whose rate is less than orequal to f_(b) bps requires only one set of demodulation circuits (amatched filter plus a RAKE demodulator), thereby making the circuitscale small.

There are two methods for thinning out the traffic channel TCH. A firstmethod makes the transmitting timings of the frames random at each basestation. A second method makes the arrangement of the data in a framerandom, and assigns the random arrangements to respective users. Therandom arrangement can be prepared, for example, from the user numberand random patterns. According to the first method, the base stationtransmits the information on TCH (traffic channel) rate to the mobilestation so that a mobile station can properly pick up data in thetraffic channel. According to the second method, it is sufficient for abase station to provide a mobile station with information on thearrangement pattern corresponding to the transmission rate.

FIG. 5 shows a major portion of a base station which employs the firstmethod. In this embodiment, the transmission rate of data inputted tochannel input terminals 111₁ -111_(n) is 1/N of the fundamentaltransmission rate, where N is a positive integer. In other words, thedata have a temporal length N times longer than the same amount of dataof the fundamental transmission rate. The data are supplied to TCH framethin-out circuits 112₁ -112_(n), and are time-compressed by a factor ofN (N=4 in FIGS. 6A-6C) at every time period T to form packets, where Tis the length of a frame of the fundamental transmission rate. Thesepackets undergo the primary modulation in primary modulators 113₁-113_(n), and then the spectrum-spreading modulation in secondarymodulators (spreading modulators) 114₁ -114_(n), thereby being convertedinto wideband signals. The spreading modulators 114₁ -114_(n) receivedifferent spreading codes C₁ -C_(n) from spreading code generators 115₁-115_(n).

In this case, the packets P₁ -P_(n) generated by the TCH frame thin-outcircuits 112₁ -112_(n) have random time relationships with each other,as shown in FIGS. 6A-6C. This is allowable because the plurality ofpackets are each associated with different spreading codes, and hence,the packets can be separated at the receiving side even if they overlapwith each other temporally. Accordingly, as soon as individual channelsignals are inputted, they can be formed into packets without any timeadjustments.

Base stations have, in addition to the traffic channels, common controlchannels for transmitting control information such as identificationinformation of respective base stations and paging information.Furthermore, a weather forecast, and other broadcasting information canbe transmitted as required. The information on the common channel isspread by spreading code C_(c), different from the spreading codes C₁-C_(n), for communications, and is transmitted from common controlchannel transmitting portion 116. The outputs of the spreadingmodulators 114₁ -114_(n+1) are combined, supplied to an output terminal117, and transmitted from a transmitter not shown in FIG. 5 as anelectric wave.

FIG. 7 shows a major portion of a mobile station, to which the presentinvention is applied. Electric waves from base stations are received bya receiving portion not shown in FIG. 7, and are inputted to an inputterminal 121 after being converted into an intermediate frequencysignal. The intermediate frequency signal is despread by a spreadingcode assigned to the communication, for example, by the spreading codeC₂. The despread output undergoes the primary demodulation by a primarydemodulator 124. The demodulated output is inputted to a frame detector125, which detects the period T and temporal positions of packets, andoutputs frame pulses as shown in FIG. 8A. The frame pulses are suppliedto a controller 126, which controls a switch 127 so that packets P₂,obtained by the despreading using the spreading code C₂, are supplied toa rate conversion circuit 128. The rate conversion circuit 128 expandsthe packets P₂ by a factor of N. Thus, the transmission data, which isinputted to the channel input terminal 111₂, and whose transmission rateis 1/N of the fundamental transmission rate, is outputted from an outputterminal 129.

The controller 126 controls the spreading code generator 123, so thatthe spreading code (C_(c), for example) associated with the commoncontrol channel of the current base station, or each of the neighboringbase stations, is outputted from the spreading code generator 123,during a porkion (for example, portion T₀₃ as shown in FIGS. 8D and 8E)other than the portion T₀₁, which receives the packet P₂ in each frameas shown in FIG. 8B. In addition, the controller 126 changes the switch127 so that the output terminal of the primary demodulator 124 isconnected to a side of a received power measurement circuit 131 and acommon control data demodulator 132. Accordingly, as shown in FIG. 8E,during the portion T₀₃ in a frame, the common control channel from abase station is received, and the received power is measured by thereceived power measurement circuit 131. Thus, the received powermeasurement is carried out in the portion T₀₃ of each frame by switchingthe spreading code to one of the spreading codes associated with thecommon control channels of the neighboring base stations. Then, the basestation having the common control channel which is associated with themaximum received power, is selected as the base station to which thecommunication is to be switched, by a base station decision circuit 133(which is usually included in the controller 126).

Changing the base station is required for continuous communication whenmeasurement of the power of the received signal from the current basestation is carried out in the portions T₀₁, and the receiving qualitydegrades below an allowable level during the communication. In thiscase, the mobile station provides, through the traffic channel, thecurrent base station with information on the base station to which thecommunication is to be switched, so that the switching to the new basestation is performed. The channel switching operation of the trafficchannel can be achieved in the same manner as a conventional method. Thepresent invention differs from the conventional method in that thedecision of the base station, to which the communication is to beswitched, is made at the mobile station side, whereas the conventionalmethod decides it at the base station side.

A common control data demodulator 132 can demodulate variousbroadcasting data or a paging during communication by supplying thedespreading circuit 122 with the spreading code associated with thecommon control channel during a portion (T₀₃, for example) other thanthe traffic packet receiving portion T₀₁. Measuring of the receivedpower and reception of the common control data can be carried out in thesame frame. For example, two identical length vacant portions T₀₃ andT₀₄ other than the packet receiving portion T₀₁ in a frame are selected,and one of them is used to measure the received power by providing thedespreading circuit 122 with the spreading code associated with thecommon control channel from a neighboring base station.

The data to be transmitted from the mobile station have a transmissionrate of 1/N of the fundamental transmission rate. A TCH frame thin-outcircuit 136 of FIG. 7 receives the data through an input terminal 135,and temporally compresses the data by a factor of N every frame intervalT to form packets as shown in FIG. 8C. The packets are assigned to avacant portion (for example, T₀₂) other than the receiving portions T₀₁and T₀₃ by the controller 126. The packets undergo the primarymodulation by a primary modulator 137. The primary modulation output isspectrum-spread modulated by a secondary modulator (spread modulator)138 using a spreading code C₂₁ from a spreading code generator 139, andthe spectrum-spread wideband signal is transmitted through a terminal141, a transmitter not shown in FIG. 7, and an antenna for bothtransmitting and receiving. Since the transmit and receive portions areseparated, the same radio carrier frequency can be used for transmitterand receiver. However, if different frequencies are used, transmittingand receiving can be performed at the same time (for example, at theportion T₀₁).

Since the data is transmitted after being time-compressed, data whosetransmission rate is from 1/N to (N-1)/N of the funaamental transmissionrate can be transmitted by using a single spreading code and by settingN equal to or greater than 3. One portion is used for receiving thebroadcasting data and the remaining (N-1) portions can be used forcommunication.

FIGS. 9A-9F illustrate the relationships between frames and packets whenthe compression ratio N=8. FIG. 9B shows the case where data whosetransmission rate is 1/8 of the fundamental transmission rate isinputted to the channel input terminal 111₁ of the transmitter of FIG.5. The data corresponding to a frame is time-compressed to a packet P₁whose length is T/8, and is transmitted. FIG. 9C shows the case wheredata whose transmission rate is 1/2 of the fundamental transmission rateis inputted to the channel input terminal 111₂ of the transmitter. Thedata corresponding to a frame is time-compressed to a packet P₂ whoselength is T/2, and is transmitted. FIG. 9D shows the case where datawhose transmission rate is 1/8 of the fundamental transmission rate isinputted to the channel input terminal 111₃ of the transmitter. The datacorresponding to a frame is time-compressed to a packet P₃ whose lengthis T/8, and is transmitted. FIG. 9E shows the case where data whosetransmission rate is 1/4 of the fundamental transmission rate isinputted to the channel input terminal 111₄ of the transmitter. The datacorresponding to a frame is time-compressed to a packet P₄ whose lengthis T/4, and is transmitted.

In this case, although the spreading codes C₁ -C_(n) inputted to thespreading modulators 114₁ -114_(n) differ from each other, the samespreading code may be consistently used for data inputted to the samechannel input terminal, or other spreading codes may be used. Forexample, the spreading code C₂ may be consistently used for the packetP₂ of FIG. 9C, or the spreading code may be changed for each one of theT/N long portions. In addition, the packet P₂ may be divided into twopackets P₂₁ and P₂₂ as shown in FIG. 9F. By thus dividing thetransmission data in a frame, an advantage of smoothing the interferenceis obtained.

The division of the transmission data in a frame can be carried out bythe following procedure.

(1) Store the input data into a memory in the TCH frame thin-outcircuit.

(2) Read data in the memory at the fundamental transmission rate in thedesignated time duration (=T/N) .

The packets shown in FIGS. 9B-9F are received by a mobile station, andrestored to the original data by the rate conversion. Thus, even if thetransmission rate of the input data is varied, the data can betransmitted as long as its transmission rate is equal to or less thanthe fundamental transmission rate. In addition, a mobile station canreceive the common control data addressed to itself or to other mobilestations in a vacant portion in each frame by switching the spreadingcode for the despreading, at timings as indicated by broken lines inFIGS. 9C-9F, for example.

Although data whose transmission rate is lower than the fundamentaltransmission rate is transmitted through a single channel in thisembodiment, the number of channels is not restricted to one. Forexample, although the packet P₂ in FIG. 9C is transmitted using four T/Nlong portions in the frame of a single channel, it can be transmitted bydistributing the packet to four channels. In this case, each channeltransmits the data using one portion in a frame.

The present invention has been described in detail with respect tovarious embodiments, and it will now be apparent from the foregoing tothose skilled in the art that changes and modifications may be madewithout departing from the invention in its broader aspects, and it isthe intention, therefore, in the appended claims to cover all suchchanges and modifications as fall within the true spirit of theinvention.

What is claimed is:
 1. A CDMA (Code Division Multiple Access)communications method for transmitting transmission data through one ormore channels between base stations and mobile stations, said methodcomprising the steps of:determining a transmission rate of thetransmission data at one of rates ranging between a value higher than apredetermined rate and a value lower than said predetermined rate;generating a frame including at least a part of said transmission dataand a vacant portion when the transmission rate of said transmissiondata is lower than the predetermined rate, said vacant portion having nodata to be transmitted; performing a primary modulation of said frame toproduce a primary modulation signal; performing a secondary modulationof said primary modulation signal using a spreading code, to produce asecondary modulated wideband signal, said spreading code being differentfor each of said channels; transmitting said wideband signal using acarrier.
 2. The CDMA communications method as claimed in claim 1,wherein said step of determining a transmission rate comprises the stepof:setting the predetermined transmission rate at a rate higher than aminimum transmission rate as a fundamental transmission rate; andwherein said step of generating a frame comprises the step of:determining a frame length such that the frame includes no vacantportion when data is transmitted at said predetermined fundamentaltransmission rate.
 3. The CDMA communication method as claimed in claim2, wherein said minimum transmission rate is a rate of transmitting avoice signal.
 4. The CDMA communications method as claimed in claim 2,further comprising the step of generating a plurality of frames to beassigned to a plurality of said channels when data is transmitted at arate higher than said predetermined fundamental transmission rate. 5.CDMA (Code Division Multiple Access) communications apparatus fortransmitting transmission data through one or more channels between basestations and mobile stations, said apparatus comprising:means fordetermining a transmission rate of the transmission data at apredetermined rate, said transmission rate of the transmission databeing variable at different rates ranging between a value higher thansaid predetermined rate and a value lower than said predetermined rate;means for generating a frame including at least a part of saidtransmission data and a vacant portion when the transmission rate ofsaid transmission data is lower than the predetermined rate, said vacantportion having no data to be transmitted; means for performing a primarymodulation of said frame to produce a primary modulation signal; meansfor performing a secondary modulation of said primary modulation signalusing a spreading code, to produce a secondary modulated widebandsignal, said spreading code being different for each of said channels;and means for transmitting said wideband signal using a carrier.
 6. TheCDMA communications apparatus as claimed in claim 5, wherein said meansfor determining a transmission rate comprises:means for setting thepredetermined transmission rate at a rate higher than a minimumtransmission rate as a fundamental transmission rate; and wherein saidmeans for generating a frame comprises: means for determining a framelength such that the frame includes no vacant portion when data istransmitted at said predetermined fundamental transmission rate.
 7. TheCDMA communications apparatus as claimed in claim 6, wherein saidminimum transmission rate is a rate of transmitting a voice signal. 8.The CDMA communications apparatus as claimed in claim 6, furthercomprising means for generating a plurality of frames to be assigned toa plurality of said channels when data is transmitted at a rate higherthan said predetermined fundamental transmission rate.
 9. In a CDMA(Code Division Multiple Access) communications system for transmittingtransmission data through one or more channels between base stations andmobile stations, a transmitter comprising:means for determining atransmission rate of the transmission data at a predetermined rate, saidtransmission rate of the transmission data being variable at differentrates ranging between a value higher than said predetermined rate and avalue lower than said predetermined rate; means for generating a frameincluding at least a part of said transmission data and a vacant portionwhen a transmission rate of said transmission date is lower than thepredetermined rate, said vacant portion having no data to betransmitted; means for performing a primary modulation of said frame toproduce a primary modulation signal; means for performing a secondarymodulation of said primary modulation signal using a spreading code, toproduce a secondary modulated signal, said spreading code beingdifferent for each of said channels; and means for transmitting saidwideband signal using a carrier.
 10. In a CDMA (Code Division MultipleAccess) communications system for transmitting transmission data throughone or more channels between base stations and mobile stations, thetransmission data being formed by a primary modulation of a frame toproduce a primary modulation signal, and a secondary modulation of theprimary modulation signal using a spreading code, to produce a secondarymodulated wideband signal, wherein a transmission rate of saidtransmission data is variable at different rates ranging a value higherthan a predetermined rate and a value lower than said predetermined rateand said transmission data has a vacant portion with no data to betransmitted when the transmission rate of said transmission data islower than said predetermined rate, a receiver for receiving thetransmission data, comprising:means for obtaining said primarymodulation signal by receiving a wideband signal associated with one ofsaid channels, and by despreading said wideband signal using a spreadingcode; and means for restoring said transmission data by primarilydemodulating said primary modulation signal obtained by said obtainingmeans, and by time-expanding a signal from said demodulating.
 11. In aCDMA (Code Division Multiple Access) communications system fortransmitting transmission data through one or more channels between basestations and mobile stations, said base stations each comprising:meansfor determining a transmission rate of the transmission data at apredetermined rate, said transmission rate of the transmission databeing variable at different rates ranging between a value higher thansaid predetermined rate and a value lower than said predetermined rate;means for generating a frame including at least a part of saidtransmission data and a vacant portion when a transmission rate of saidtransmission data is lower than said predetermined rate, said vacantportion having no data to be transmitted; means for performing a primarymodulation of said frame to produce a primary modulation signal; meansfor performing a secondary modulation of said primary modulation signalusing a spreading code, to produce a secondary modulated widebandsignal, said spreading code being different for each of said channels;and means for transmitting said wideband signal using a carrier.
 12. Ina CDMA (Code Division Multiple Access) communications system fortransmitting transmission data through one or more channels between basestations and mobile stations, the transmission data being formed by aprimary modulation of a frame to produce a primary modulation signal,and a secondary modulation of the primary modulation signal using aspreading code, to produce a secondary modulated wideband signal,wherein a transmission rate of said transmission data is variable atdifferent rates ranging between a value higher than a predetermined rateand a value lower than the predetermined rate and said transmission datahas a vacant portion with no data to be transmitted when thetransmission rate of said transmission data is lower than thepredetermined rate, said mobile stations each comprising:means forobtaining said primary modulation signal by receiving a wideband signalassociated with one of said channels, and by despreading said widebandsignal using a spreading code; and means for restoring said transmissiondata by primarily demodulating said primary modulation signal obtainedby said obtaining means, and by time-expanding a signal from saiddemodulating.
 13. A CDMA (Code Division Multiple Access) communicationssystem for transmitting transmission data through one or more channelsbetween base stations and mobile stations, said system comprising:meansfor determining a fundamental transmission rate of the transmission dataat a predetermined rate lower than a minimum transmission rate, meansfor selecting said transmission rate of the transmission data within arange between a value higher than said predetermined fundamentaltransmission rate and a value lower than said predetermined fundamentaltransmission rate; a first frame generator for generating a framewithout a vacant portion, said vacant portion having no data to betransmitted, when the transmission rate of said transmission data is atthe predetermined fundamental transmission rate; a second framegenerator for generating a frame including at least a part of saidtransmission data and a vacant portion with no data to be transmitted,when the transmission rate of said transmission data is lower than thepredetermined fundamental transmission rate, said vacant portion havingno data to be transmitted; a third frame generator for generating aplurality of frames including the transmission data, when thetransmission rate of said transmission data is higher than thepredetermined fundamental transmission rate, a thin-out circuitconnected to the second frame generator for thinning out the frame toform the vacant portion, when the transmission rate of said transmissiondata is lower than the predetermined fundamental transmission rate, anda multiple modulator for modulating the frames using a spreading code,to produce a modulated wideband signal, said spreading code beingdifferent for each of said channels, when the transmission rate of saidtransmission data is higher than the predetermined fundamentaltransmission rate.
 14. The CDMA (Code Division Multiple Access)communications system as claimed in claim 13 further comprising:meansfor measuring received power of a common control channel signaltransmitted from a base station other than a base station with which themobile station is communicating, by switching said spreading code duringa time period corresponding to said vacant portion in said frame. 15.The CDMA (Code Division Multiple Access) communications system asclaimed in claim 13 further comprising:means for receiving commoncontrol channel data transmitted from at least one of said base stationsduring a time period corresponding to said vacant portion of said frameby switching said spreading code.